Generally the composition and characteristics of coal can be described as relative amounts of moisture, volatiles, fixed carbon and ash. In describing coal the industry has standardized on data from basic tests and procedures. For example, the moisture content of coal is determined by subjecting the coal as received to heat under standard conditions with the temperature maintained slightly above the boiling point of water. This procedure results in drying of the coal and a resultant loss of weight which is readily measurable. This simple test provides a reasonably accurate measure of water entrained in the coal, although it is recognized that further heating at higher temperature could result in the expulsion of greater amounts of moisture. Likewise, the industry has standardized on tests and procedures for determining the volatile content of coal. After drying the coal to determine moisture content as described above, the dried coal is placed in a closed container where it is heated for a specific time, for example 7 minutes at an elevated temperature, for example 950.degree. C (1742.degree. F). Thus the volatile matter in coal can be determined by measuring the loss in weight, although it is recognized that the amount of volatile matter given up by the coal would change should the length of heating time be changed, the temperature be changed, or both. Further the standard tests may be continued by taking the residual solid material and burning it under standard conditions to a final residual or ash. Then by adding up the relative amounts of moisture, volatiles and ash expressed as percentages and subtracting the total from 100, the relative amount of fixed carbon can be computed.
The volatile matter in coal is not truly volatile in the strictest sense, but rather volatiles are a result of decomposition of the coal when subjected to heat. Volatiles extracted from coal include for the most part combustible gases, with smaller amounts of non-combustible gases. Among the combustibles are numerous hydrocarbons (including methane), hydrogen, carbon monoxide and the like. Non-combustibles generally are water vapor, carbon dioxide and the like. Further, it is quite common to find combustible gases entrained in the coal apart from the so called volatiles. Many coal deposits have large quantities of entrained combustible gases, commonly called "fire damp," the principal constituent of which is methane. In this regard it is not uncommon among coal deposits in the United States to find coal beds that contain in the order to 100 standard cubic feet of methane entrained in each ton of coal in place. Methane entrained in coal compares favorably to natural gas of petroleum origin and may be recovered, in part, from coal by the simple expedient of drilling a well from the surface of the ground into the coal deposit. While methane may be recovered from coal in this manner, rarely is it commercially attractive to do so because the methane in coal is under moderate pressure compared to methane of petroleum origin, and the resultant flow rates to the well bore are quite low, the captured gas at the surface must be compressed in order to be moved by pipeline, and the like. Methane entrained in coal cannot be removed entirely by pressure differential without introducing another fluid to displace the methane.
In the coal bearing regions of the world it is quite common to find multibedded coal deposits in which in vertical sequence and in descending order there is the overburden, then a bed of coal, then a layer of sedimentary rock, then a bed of coal, then a layer of sedimentary rock, then a bed of coal, and so on. In some cases the various beds of coals may be separated by only a short distance such as 1 to 5 feet. In other cases the beds of coal may be separated by greater distances, for example 50 to 300 feet. Generally one bed of the sequence is of particular interest because of its areal extent, the quality of the coal, its bed thickness and the like. Nearby beds may not be of commercial interest because the seam is too thin for standard mining equipment, the coal contains too much debris, and similar factors. In these cases the beds of commercial interest are produced by conventional mining methods while nearby beds of coal remain untouched because the cost of extraction exceeds the market value of recovered coal.
Looking to newer methods of producing coal and in particular to the gasification of coal in situ, economic evaluation of a multibedded coal deposit also is required before production begins. As in the case of conventional mining of coal, thickness of the coal bed is a critical consideration. Factors that are detrimental to conventional mining of coal -- increasingly thickening overburdens, high moisture contents, high ash contents, high firedamp contents, and the like -- often are advantages to production of coal in situ by gasification. Generally, coal beds that are of the proper thickness for conventional mining are also of acceptable thickness for in situ gasification. Coal beds that are too thin for conventional mining, generally also are too thin for in situ gasification. Thus thin beds of coal remain unproduced when they overlie or underlie coal beds that are being produced by methods heretofore known.